Diversity receiver

ABSTRACT

A diversity receiver includes a plurality of antennas to receive radio frequency signals. A plurality of receiver circuits are each coupled to a respective one of the plurality of antennas to process the received radio frequency signals, and a channel estimator is coupled to at least one of the receiver circuits to determine at least one of channel estimation values for the received radio frequency signals. A controller is coupled to the channel estimator and to at least one of the receiver circuits and selectively activates or deactivates the at least one receiver circuit based on the determined at least one channel estimation value.

RELATED APPLICATIONS

This application is a Divisional application of co-pending U.S. patentapplication Ser. No. 11/510,318, which has a filing date of Aug. 25,2006. The entire contents of the co-pending application are incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to receivers in general and more particularly todiversity receivers.

BACKGROUND OF THE INVENTION

In radio frequency transmission systems, signals that are transmittedbetween a transmitter and a receiver can deteriorate or be lost due tomulti-path fading or shadowing. In these cases, diversity receivers mayprovide an improvement. Diversity receivers comprise two or moreseparate receiver circuits, each with its own antenna, and a combinerthat combines the signals received by the individual antennas. Since thesignals propagate from the transmitter to the individual antennas viadifferent transmission channels and since each of the transmissionchannels experiences different multi-path fading and shadowing, a moreaccurate signal can be produced when combining the signals received bythe individual antennas.

In one of many ways, diversity receivers receive signals which areso-called multi-carrier signals. Multi-carrier signals are produced bysplitting a signal to be transmitted into a plurality of sub-signals,each of which is transmitted separately on an individual frequencycarrier. A receiver receives the sub-signals from each of the carriersand recombines them to reproduce the original signal. In multi-carriertransmission systems, diversity receivers take advantage of the factthat the multi-path fading and shadowing on the different antennas isnot the same so that, when one antenna receives a multi-carrier signalthat comprises faded sub-signals, chances are that another antennareceives these sub-signals without fading. Combining the multi-carriersignals received by the individual antennas can thus mitigate fading.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a diversity receivercomprises a plurality of antennas, a plurality of receiver circuits, achannel estimator and a controller. The antennas receive radio frequencysignals. Each of the receiver circuits is coupled to one of the antennasto process the received radio frequency signals. The channel estimatoris coupled to at least one of the receiver circuits and determineschannel estimation values for the received radio frequency signals. Thecontroller is coupled to the channel estimator and to at least one ofthe receiver circuits. The controller selectively activates anddeactivates the at least one receiver circuit depending on thedetermined channel estimation values.

According to another embodiment of the invention, a diversity receivercomprises a plurality of antennas, a plurality of receiver circuits, aDoppler frequency shift calculator and a controller. The antennasreceive radio frequency signals. Each of the receiver circuits iscoupled to one of the antennas to process the received radio frequencysignals. The Doppler frequency shift calculator is coupled to at leastone of the receiver circuits and determines Doppler frequency shiftvalues for the received radio frequency signals. The controller iscoupled to the Doppler frequency shift calculator and to at least one ofthe receiver circuits. The controller selectively activates anddeactivates the at least one receiver circuit depending on thedetermined Doppler frequency shift values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a diversity receiver 100 according to afirst exemplary embodiment of the invention.

FIG. 2 schematically illustrates a further diversity 200 receiveraccording to a second exemplary embodiment of the invention.

FIG. 3 schematically illustrates a further diversity receiver 300according to a third exemplary embodiment of the invention.

FIG. 4 schematically illustrates channel estimation values for severalsub-carriers of a multi-carrier transmission system.

FIG. 5 schematically illustrates a further diversity receiver 500according to a fourth exemplary embodiment of the invention.

FIG. 6 schematically illustrates a further diversity receiver 600according to a fifth exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, one or more aspects and/or embodiments of theinvention are described with reference to the drawings, wherein likereference numerals are generally utilized to refer to like elementsthroughout, and wherein the various structures are not necessarily drawnto scale. In the following description, for purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of one or more aspects of embodiments of the invention. Itmay be evident, however, to one skilled in the art that one or moreaspects of the embodiments of the invention may be practiced with alesser degree of these specific details. In other instances, knownstructures and devices are shown in block diagram form in order tofacilitate describing one or more aspects of the embodiments of theinvention. The following description is therefore not to be taken in alimiting sense, and the scope of the invention is defined by theappended claims.

Referring to FIG. 1, a block diagram of a diversity receiver 100 isshown which serves as an exemplary embodiment of one aspect of theinvention. The diversity receiver 100 comprises a plurality of antennas101 ₁, 101 ₂, . . . , 101 _(n), a corresponding number of receivercircuits 102 ₁, 102 ₂, . . . , 102 _(n), a channel estimator 103 and acontroller 104.

Each receiver circuit 102 ₁, 102 ₂, . . . , 102 _(n) comprises an inputterminal, an output terminal and a control terminal. The channelestimator 103 comprises input terminals and an output terminal. Thecontroller 104 comprises an input terminal and output terminals.

Each antenna 101 ₁, 101 ₂, . . . , 101 _(n) is coupled to the inputterminal of one of the receiver circuits 102 ₁, 102 ₂, . . . , 102 _(n).Each antenna 101 ₁, 101 ₂, . . . , 101 _(n) together with thecorresponding receiver circuit 102 ₁, 102 ₂, . . . , 102 _(n) forms adiversity branch of the diversity receiver 100. The diversity receiver100 comprises at least two diversity branches (n≧2), which are in oneembodiment functionally equivalent.

The output terminal of at least one of the receiver circuits 102 ₁, 102₂, . . . , 102 _(n) is connected to one of the input terminals of thechannel estimator 103. It can also be provided that the output terminalsof all receiver circuits 102 ₁, 102 ₂, . . . , 102 _(n) are connected tothe input terminals of the channel estimator 103 as shown in FIG. 1. Theoutput terminal of the channel estimator 103 is connected to the inputterminal of the controller 104. At least one output terminal of thecontroller 104 is connected to the control terminal of one of thereceiver circuits 102 ₁, 102 ₂, . . . , 102 _(n). It may also beprovided that each output terminal of the controller 104 is connected toone of the control terminals of the receiver circuits 102 ₁, 102 ₂, . .. , 102 _(n) as shown in FIG. 1.

During the use of the diversity receiver 100, the antennas 101 ₁, 101 ₂,. . . , 101 _(n) receive radio frequency signals and transfer thereceived radio frequency signals to the receiver circuits 102 ₁, 102 ₂,. . . , 102 _(n), respectively. The receiver circuits 102 ₁, 102 ₂, . .. , 102 _(n) process the received radio frequency signals and at leastone of the receiver circuits 102 ₁, 102 ₂, . . . , 102 _(n) feeds itsprocessed signals to the channel estimator 103. The channel estimator103 determines channel estimation values for the received radiofrequency signals. The channel estimation values are fed to thecontroller 104. The controller 104 selectively activates and deactivatesat least one of the receiver circuits 102 ₁, 102 ₂, . . . , 102 _(n) viaits control terminals depending on the determined channel estimationvalues.

In FIG. 1 a single channel estimator 103 determines the channelestimation values for all diversity branches or a number of thediversity branches. This is not to be taken in a limiting sense. Thechannel estimator 103 may for example also be comprised by the receivercircuits 102 ₁, 102 ₂, . . . , 102 _(n). For example, each diversitybranch may comprise a separate channel estimator which determines thechannel estimation values for the radio frequency signals received bythat diversity branch.

Referring to FIG. 2, a block diagram of a diversity receiver 200 isshown which serves as an exemplary embodiment of a further embodiment ofthe invention. The diversity receiver 200 comprises a plurality ofantennas 201 ₁, 201 ₂, . . . , 201 _(n), a corresponding number ofreceiver circuits 202 ₁, 202 ₂, . . . , 202 _(n), a Doppler frequencyshift calculator 203 and a controller 204. The wiring of the componentsof the diversity receiver 200 is equivalent to the wiring of thecomponents of the diversity receiver 100 shown in FIG. 1, wherein theexternal wiring of the Doppler frequency shift calculator 203corresponds to the external wiring of the channel estimator 103. Thediversity receiver 200 comprises at least two diversity branches (n≧2).

During the use of the diversity receiver 200 the antennas 201 ₁, 201 ₂,. . . , 201 _(n) receive radio frequency signals and transfer thereceived radio frequency signals to the receiver circuit 202 ₁, 202 ₂, .. . , 202 _(n), respectively. The receiver circuits 202 ₁, 202 ₂, . . ., 202 _(n) process the received radio frequency signals and at least oneof the receiver circuits 202 ₁, 202 ₂, . . . , 202 _(n) feeds itsprocessed signals to the Doppler frequency shift calculator 203. TheDoppler frequency shift calculator 203 determines Doppler frequencyshift values for the received radio frequency signals. The Dopplerfrequency shift values are fed to the controller 204. The controller 204selectively activates and deactivates at least one of the receivercircuits 202 ₁, 202 ₂, . . . , 202 _(n) depending on the determinedDoppler frequency shift values.

Analogously to the channel estimator 103 shown in FIG. 1, the Dopplerfrequency shift calculator 203 can for example also be implemented inthe receiver circuits 202 ₁, 202 ₂, . . . , 202 _(n) or a number of thereceiver circuits 202 ₁, 202 ₂, . . . , 202 _(n).

The diversity receivers 100 and 200 shown in FIGS. 1 and 2 may forexample comprise all features or a selection of the features of thediversity receiver 300, which is described in the following.

A block diagram of the diversity receiver 300 is shown in FIG. 3. Thediversity receiver 300 serves as another exemplary embodiment of theinvention. The diversity receiver 300 comprises two diversity branches,but may also comprise more than two diversity branches. The diversitybranches are functionally equivalent. One of the diversity branches,which is called upper diversity branch in the following, comprises anantenna 301 ₁, a mixer circuit 302 ₁, an analog-to-digital converter 303₁ and a demodulator 304 ₁. The other diversity branch, which is calledlower diversity branch in the following, comprises an antenna 301 ₂, amixer circuit 302 ₂, an analog-to-digital converter 303 ₂ and ademodulator 304 ₂. Furthermore, the diversity receiver 300 comprises anoptional antenna switch 305, a controller 306, a combiner 307 and aforward error correction unit 308.

The antennas 301 ₁ and 301 ₂ are coupled to the respective mixercircuits 302 ₁ and 302 ₂ via the antenna switch 305. According to oneembodiment of the invention, a low noise amplifier is connected betweeneach antenna 301 ₁ and 301 ₂ and the antenna switch 305. The low noiseamplifiers are not shown in FIG. 3. The mixer circuits 302 ₁ and 302 ₂down-convert the radio frequency signals received on the antennas 301 ₁and 301 ₂, respectively. The down-converted signals are fed to theanalog-to-digital converters 303 ₁ and 303 ₂, respectively, to generatesampled signals. The demodulators 304 ₁ and 304 ₂ produce demodulatedsignals out of the sampled signals, respectively.

The demodulated signals produced by the demodulators 304 ₁ and 304 ₂ aretransferred to the combiner 307. In FIG. 3 the combiner 307 is part ofthe upper diversity branch. Alternatively, the combiner 307 could bepart of another diversity branch or could be separated from thediversity branches. The combiner 307 combines the demodulated signals inorder to provide a composite signal with better signal quality thaneither signal alone. Combining can be carried out by using a commoncombining method, such as maximum ratio combining, carrier selection,equal gain combining or any other form of combining. The compositesignal generated by the combiner 307 is input into the forward errorcorrection unit 308. After the forward error correction the compositesignal is provided for further processing.

Since the radio frequency signals received on the antennas 301 ₁ and 301₂ may be distorted due to multi-path propagation between a transmittertransmitting the radio frequency signals and the diversity receiver 300,it is intended according to one embodiment to correct the receivedsignals. The correction mechanism is based on continuously repeatedmeasurement of the channel characteristics of the transmission channelsvia which the radio frequency signals propagate to the antennas 301 ₁and 301 ₂ (channel estimation). The information which is determinedabout the transmission channels during the channel estimation process isused for equalization of the received signals.

In the present exemplary embodiment of the invention each demodulator304 ₁ and 304 ₂ comprises a channel estimator. The channel estimatorscould for example also be separated from the demodulators 304 ₁ and 304₂ or, alternatively, there could be a single channel estimator commonlyused for both diversity branches.

In order to allow channel estimation in the diversity receiver 300, thetransmitter transmits according to one embodiment pilot symbols whichare known in the diversity receiver 300. The diversity receiver 300receives the distorted pilot symbols, which are transmitted via the sametransmission channels than the payload data, and compares them with theknown pilot symbols. The quotient of the pilot symbols as received via aspecific propagation path and the known pilot symbols then for exampleresults in a channel estimation value, which is also known in the art aschannel coefficient. The channel estimation value corresponds to thetransfer function of the transmission channel at a given carrierfrequency used for signal transmission. The channel estimation valuesallow to compensate the received signals for the rotation and magnitudechange which occurred to the complex symbols in the transmission pathresulting in a lower bit error rate.

According to one embodiment, the channel estimation values are used todecide whether it is necessary to use all of the diversity branches at aparticular time or whether one of the diversity branches can bedeactivated in order to reduce the overall power consumption of thediversity receiver 300. For example, if the channel estimation valuesindicate that the quality of the transmission channels is high, it isnot necessary to use both of the diversity branches of the diversityreceiver 300 and it can be decided that one diversity branch isdeactivated. Once the quality of the transmission channel of theremaining active diversity branch gets worse, the deactivated diversitybranch may be activated again. Proceeding this way is advantageous inthe sense that power saving can be achieved during the time one of thediversity branches is deactivated. This is especially beneficial if thediversity receiver 300 is implemented into a cellular phone or any othermobile device.

The selective deactivation and activation of diversity branches iscarried out by the controller 306. For that reason, the controller 306receives the determined channel estimation values from the demodulators304 ₁ and 304 ₂. The controller 306 selectively deactivates a diversitybranch for example by turning off its power supply circuit or bydisconnecting the diversity branch from the power supply circuit.

Many configurations are possible how the activation and deactivation ofat least one of the diversity branches could be implemented in thediversity receiver 300 comprising two or more diversity branches.According to one embodiment, decision is made when the performance gainfrom using diversity outweighs the extra power consumption. For example,if the channel estimation values are higher than a predeterminedthreshold, it may be sufficient to use only one or a few of thediversity branches. All other diversity branches or a number of them canbe deactivated. When the channel estimation values of the remainingactive diversity branches drop below the same or a further predeterminedthreshold, the deactivated diversity branches or a predetermined numberof them can be activated again.

Furthermore, before deactivating one or a number of the diversitybranches, the diversity branch or branches can, for example, bedetermined that exhibit the best channel estimation values. Thesediversity branches can be selected for receiving the radio frequencysignals while the other diversity branches are deactivated. For thisreason, the diversity receiver 300 comprises the antenna switch 305which is arranged between the antennas 301 ₁ and 301 ₂ and the mixercircuits 302 ₁ and 302 ₂. If it is found, for example, that the antenna301 ₂ receives radio frequency signals with a better quality than theantenna 301 ₁ and the quality of the radio frequency signals received onthe antenna 301 ₂ is sufficiently high, the antenna switch 305 canconnect the antenna 301 ₂ to the mixer circuit 302 ₁ and the controller306 can deactivate the mixer circuit 302 ₂, the analog-to-digitalconverter 303 ₂ and the demodulator 304 ₂.

If the diversity receiver 300 comprises more than two diversitybranches, it can, for example, be provided that one or more diversitybranches are deactivated when the channel estimation values of apredetermined number of diversity branches cross a predeterminedthreshold. If later the channel estimation values of a predeterminednumber of the remaining active diversity branches drop below apredetermined threshold, all deactivated diversity branches or a numberof them can be activated again.

It can, for example, also be provided that deactivating and activatingdiversity branches is carried out in a staggered manner by using aplurality of thresholds. For example, if the channel estimation valuesof a predetermined number of diversity branches cross a first threshold,a predetermined number of diversity branches will be deactivated. If thechannel estimation values of a predetermined number of the remainingactive diversity branches cross a second threshold which is higher thanthe first threshold, a predetermined number of the remaining activediversity branches will be deactivated as well. This proceeding can becontinued by using an arbitrary number of thresholds. Analogously, aplurality of thresholds can, for example, also be provided foractivating the diversity branches.

For the data transmission between the transmitter and the diversityreceiver 300 any possible data transmission standards can be used, suchas GSM (global system for mobile communications) or UMTS (universalmobile telecommunications system) for example. Furthermore, the signalsthat are transmitted between the transmitter and the diversity receiver300 can be multi-carrier signals. In this case, for example, OFDM(orthogonal frequency division multiplex) can be used as a modulationmethod. Moreover, the transmission system can, for example, be used forthe broadcast transmission of digital terrestrial television. OFDM hasbeen adopted as the modulation method in a number of systems for digitalterrestrial television, such as DVB-T (digital videobroadcasting-terrestrial), which can for example be used for thediversity receiver 300 as well.

If the transmitted radio frequency signals are multi-carrier signals,there are even more possibilities how the deactivation and activation ofat least one of the diversity branches can be accomplished. This isbecause channel estimation values can be determined for the transmissionchannels of each sub-carrier to each antenna 301 ₁ and 301 ₂. A channelestimation value of a sub-carrier corresponds to the transfer functionof the respective transmission channel of the sub-carrier at thefrequency of the sub-carrier.

FIG. 4 shows a diagram in which channel estimation values H(μ) areplotted versus frequency f. The frequencies f are the carrierfrequencies of the sub-signals of a multi-carrier signal. In oneembodiment, one of the antennas 301 ₁ and 301 ₂ will be deactivated ifthe channel estimation values of a predetermined number of thesub-signals of a multi-carrier signal received on one or both antennas301 ₁ and 301 ₂ is higher than a threshold level 400 shown in FIG. 4.Analogously, if the channel estimation values of a predetermined numberof the sub-signals of a multi-carrier signal received on the remainingactive antenna 301 ₁ or 301 ₂ is smaller than a further threshold level,the deactivated diversity branch 301 ₁ or 301 ₂ will be returned to itsactive mode. The aforementioned criteria on how to selectively activateand deactivate at least one diversity branch of the diversity receiver300 in case the transmitted signals are multi-carrier signals can becombined with any other criterion described above.

According to another embodiment of the invention Doppler frequency shiftvalues instead of channel estimation values are used to decide whetherone or more of the diversity branches shall be deactivated. In a mobilecommunication system, a Doppler frequency shift occurs when the velocityvector of the transmitter differs from the velocity vector of thereceiver. In this case the transmitter transmits signals with afrequency f₀, whereas the actual frequency at the receiver is f₀+Δf,wherein Δf is the Doppler frequency shift. The Doppler frequency shiftusually reduces the probability to detect transmitted data correctly.

According to one embodiment of the invention, the controller 306 of thediversity receiver 300 determines Doppler frequency shift values thatcharacterize the Doppler frequency shift of the radio frequency signalsreceived by the diversity receiver 300. The controller 306 for exampleactivates all or a predetermined number of diversity branches if theDoppler frequency shift values are greater than a predeterminedthreshold, which is indicative of a certain relative speed of thediversity receiver 300 with respect to the transmitter. If thetransmitter and the diversity receiver 300 are however stationary withrespect to each other, the Doppler frequency shift is zero and thediversity may be reduced. Therefore it may be provided, for example,that at least one of the diversity branches will be deactivated if theDoppler frequency shift values drop below a predetermined threshold.This procedure helps to reduce the overall power consumption of thediversity receiver 300.

It may for, example, also be provided that deactivating and activatingdiversity branches is carried out in a staggered fashion by using aplurality of thresholds. For example, if the Doppler frequency shiftvalues become greater than a first threshold, a predetermined number ofdeactivated diversity branches will be activated. If the Dopplerfrequency shift values cross a second threshold, a further predeterminednumber of the deactivated diversity branches will be activated. Thisproceeding can be continued by using a non-limited number of thresholds.Analogously, a plurality of thresholds can also be provided for thedeactivation of the diversity branches.

The diversity receiver 300 may, for example, determine the Dopplerfrequency shift values by measuring the rate with which the envelope ofthe received radio frequency signals cross a predetermined thresholdlevel. The level crossing rate is proportional to the Doppler frequencyshift. Another method for determining the Doppler frequency shift valuesis based on measuring the zero crossing rate of the in-phase orquadrature part of the received radio frequency signals.

The criterion according to which selective activation and deactivationof diversity branches is carried out depending on the Doppler frequencyshift values can, for example, be combined with the criterion based onthe channel estimation values as described above. Furthermore, otherparameters can also contribute to the decision when antenna diversity isjustified in light of the additional power consumption thereof. Forexample, a low battery power in a mobile device or a highsignal-to-noise ratio or a high signal strength of the received radiofrequency signals may lead to the decision to reduce diversity and toreduce the number of active diversity branches. Signals indicating thesignal-to-noise ratio or the signal strength of the received radiofrequency signals can for example be provided by the mixer circuits 302₁ and 302 ₂ to the controller 306.

According to one embodiment of the invention, operational modes of thediversity receiver 300 can be selected which are independent of thedetermined channel estimation values or Doppler frequency shift values.In one operational mode, a minimum number of diversity branches, forexample only a single diversity branch, is active. This operational modemay, for example, be selected if the battery power is low. In anotheroperational mode, a maximum number of diversity branches, for exampleall of the diversity branches, are active.

The activation and deactivation of diversity branches can in principlebe carried out after every transmission of a symbol. Usually, however,longer time intervals are kept between points in time when diversitybranches are activated or deactivated. The present inventioncontemplates both situations.

Referring to FIG. 5, a block diagram of a diversity receiver 500 isshown which serves as another exemplary embodiment of the invention. Thediversity receiver 500 comprises a plurality of antennas 501 ₁, 501 ₂, .. . , 501 _(n), a corresponding number of receiver circuits 502 ₁, 502₂, . . . , 502 _(n), a combiner 503 and a controller 504.

Each antenna 501 ₁, 501 ₂, . . . , 501 _(n) is coupled to an inputterminal of one of the receiver circuits 502 ₁, 502 ₂, . . . , 502 _(n).Each antenna 501 ₁, 501 ₂, . . . , 501 _(n) together with thecorresponding receiver circuit 502 ₁, 502 ₂, . . . , 502 _(n) forms adiversity branch of the diversity receiver 500. Each receiver circuit502 ₁, 502 ₂, . . . , 502 _(n) comprises a channel estimator whichdetermines channel estimation values 505 ₁, 505 ₂, . . . , 505 _(n) forthe radio frequency signals received by the corresponding antenna 501 ₁,501 ₂, . . . , 501 _(n). The receiver circuits 502 ₁, 502 ₂, . . . , 502_(n) use the determined channel estimation values 505 ₁, 505 ₂, . . . ,505 _(n) to extract transmitted data 506 ₁, 506 ₂, . . . , 506 _(n) fromthe received radio frequency signals, respectively.

The extracted data 506 ₁, 506 ₂, . . . , 506 _(n) and the determinedchannel estimation values 505 ₁, 505 ₂, . . . , 505 _(n) are transferredto the combiner 503. The combiner 503 combines the data 506 ₁, 506 ₂, .. . , 506 _(n). Combining can be carried out by using a common combiningmethod, such as maximum ratio combining, carrier selection, equal gaincombining or any other form of combining. Combined data 507 are providedat an output terminal of the combiner 503 for further processing.

The channel estimation values 505 ₁, 505 ₂, . . . , 505 _(n) are alsofed to the controller 504. The controller 504 selectively activates anddeactivates the receiver circuits 502 ₁, 502 ₂, . . . , 502 _(n) viacontrol signals 508 ₁, 508 ₂, . . . , 508 _(n) depending on thedetermined channel estimation values 505 ₁, 505 ₂, . . . , 505 _(n).Furthermore, the controller 504 controls the combiner 503 via a controlsignal 509.

Referring to FIG. 6, a block diagram of a diversity receiver 600 isshown which serves as another exemplary embodiment of the invention. Thediversity receiver 600 comprises a plurality of antennas 601 ₁, 601 ₂, .. . , 601 _(n), a corresponding number of receiver circuits 602 ₁, 602₂, . . . , 602 _(n), a combiner 603 and a controller 604. The wiring andthe function of the diversity receiver 600 is almost identical to thewiring and the function of the diversity receiver 500 shown in FIG. 5.The difference between the diversity receivers 500 and 600 is that thecontroller 604 uses Doppler frequency shift values 610 ₁, 610 ₂, . . . ,610 _(n) instead of channel estimation values 605 ₁, 605 ₂, . . . , 605_(n) in order to control the activation and deactivation of the receivercircuits 602 ₁, 602 ₂, . . . , 602 _(n). The Doppler frequency shiftvalues 610 ₁, 610 ₂, . . . , 610 _(n) are generated by the receivercircuits 602 ₁, 602 ₂, . . . , 602 _(n) and transferred to thecontroller 604.

The diversity receivers 500 and 600 shown in FIGS. 5 and 6 may forexample comprise all features or a selection of the features of thediversity receiver 300 which was described above.

In addition, while a particular feature or aspect of an embodiment ofthe invention may have been disclosed with respect to only one ofseveral implementations, such feature or aspect may be combined with oneor more other features or aspects of the other implementations as may bedesired and advantageous for any given or particular application.Furthermore, to the extent that the terms “include”, “have”, “with”, orother variants thereof are used in either the detailed description orthe claims, such terms are intended to be inclusive in a manner similarto the term “comprise”. The terms “coupled” and “connected”, along withderivatives may have been used. It should be understood that these termsmay have been used to indicate that two elements co-operate or interactwith each other regardless whether they are in direct physical orelectrical contact, or they are not in direct contact with each other.Furthermore, it should be understood that embodiments of the inventionmay be implemented in discrete circuits, partially integrated circuitsor fully integrated circuits or programming means. Also, the term“exemplary” is merely meant as an example, rather than the best oroptimal. It is also to be appreciated that features and/or elementsdepicted herein are illustrated with particular dimensions relative toone another for purposes of simplicity and ease of understanding, andthat actual dimensions may differ substantially from that illustratedherein.

1. A diversity receiver, comprising: a plurality of antennas configuredto receive radio frequency signals; a plurality of receiver circuitseach coupled to one of the antennas and configured to process thereceived radio frequency signals; a Doppler frequency shift calculatorcoupled to at least one of the receiver circuits and configured todetermine at least one of Doppler frequency shift value for therespective received radio frequency signals; and a controller coupled tothe Doppler frequency shift calculator and to at least one of thereceiver circuits, and configured to selectively activate and deactivatethe at least one receiver circuit based on the determined at least oneof Doppler frequency shift value.
 2. The diversity receiver of claim 1,wherein the controller is configured to deactivate the at least onereceiver circuit if the determined at least one of Doppler frequencyshift value exceeds a predetermined threshold.
 3. The diversity receiverof claim 2, wherein the controller is configured to activate the atleast one receiver circuit if the determined at least one of Dopplerfrequency shift value exceeds a further predetermined threshold.
 4. Thediversity receiver of claim 1, wherein a first plurality of the antennasand the receiver circuits coupled to the first plurality of antennasform a first group, and the Doppler frequency shift calculator coupledis coupled to the receiver circuits of the first group and is configuredto determine Doppler frequency shift values for the radio frequencysignals received by the antennas of the first group.
 5. The diversityreceiver of claim 4, wherein a plurality of the receiver circuits form asecond group, and the controller is coupled to the receiver circuits ofthe second group and is configured to deactivate the receiver circuitsof the second group if a predetermined number of the determined Dopplerfrequency shift values for the radio frequency signals received by apredetermined number of antennas of the first group exceeds apredetermined threshold.
 6. The diversity receiver of claim 1, whereinan operational mode of the diversity receiver is selectable in which apredetermined number of the receiver circuits is activated based on theselected operational mode.
 7. The diversity receiver of claim 1, whereinthe radio frequency signals received by the antennas are multi-carriersignals.
 8. A diversity receiver, comprising: a plurality of antennasconfigured to receive radio frequency signals; a plurality of receivercircuits for processing the received radio frequency signals; a Dopplerfrequency shift calculator for determining at least one Dopplerfrequency shift value for the respective received radio frequencysignals; and a controller for selectively activating and deactivating atleast one of the plurality of receiver circuits for processing thereceived radio frequency signals based on the determined at least oneDoppler frequency shift value.
 9. The diversity receiver of claim 8,wherein the controller is configured to deactivate the at least onereceiver circuit if the determined the Doppler frequency shift valueexceeds a predetermined threshold.
 10. The diversity receiver of claim8, wherein a first plurality of the antennas and the receiver circuitscoupled to the first plurality of antennas form a first group, and theDoppler frequency shift calculator is coupled to the receiver circuitsof the first group and is configured to determine Doppler frequencyshift values for the radio frequency signals received by the antennas ofthe first group.
 11. The diversity receiver of claim 8, wherein anoperational mode of the diversity receiver is selectable in which apredetermined number of the receiver circuits is activated based on theselected operational mode.
 12. The diversity receiver of claim 8,wherein the radio frequency signals received by the antennas aremulti-carrier signals.
 13. A method, comprising: receiving radiofrequency signals on a plurality of antennas; processing the receivedradio frequency signals using a plurality of receiver circuits, whereinthe radio frequency signals received on each antenna are processed in anindividual receiver circuit, respectively; determining at least oneDoppler frequency shift value for the radio frequency signals receivedon at least one of the antennas; and selectively activating ordeactivating at least one of the receiver circuits based on thedetermined at least one Doppler frequency shift value.
 14. The method ofclaim 13, wherein the at least one receiver circuit is deactivated ifthe determined at least one Doppler frequency shift value crosses apredetermined threshold in a predetermined direction.
 15. The method ofclaim 14, wherein the at least one receiver circuit is activated if thedetermined at least one Doppler frequency shift value crosses a furtherpredetermined threshold in a direction opposite to the predetermineddirection.